Lets take this C code. Obviously, it will always print out normal value
int main() {
for(int i=0; i<2; i++) {
if(i>4)
put(“High value”)
else
put(“Normal value”)
}
return 0;
}
Lets say I want the first time it runs this loop to go through the normal control flow. I got some simple code when compiling with -S -emit-llvm . Lets say after it loops I’d like to go to the if statement that puts high value instead of the start of the for loop.
I was thinking this phi statement would work. The bottom of the for loop is label %14, 0% is the obviously the first label/control block, In the phi statement I use %6 (start of for body) and %9 (the block with puts high)
%a = phi label [%6, %0], [%9, %14]
Then I replace the %6 with %a
br i1 %5, label %6, label %17 ;17 is after the foor loop, the return 0 line
I get the error “error: expected a basic block”. I’m assuming it’s because %a isn’t a fixed value.
What my real goal is in the middle of the for loop I’d like to re-use the loop increment and conditional and continue on. Some conditions can be quite long (a&&b && (c||d) && e>f && more) so it seems like generating the condition at every point would cause a lot of code and be slow until I hit it with an optimizer. What are my options? Can I do this without changing it to a large switch statement?